1. Field of the Invention
The present invention relates to a computer system, and, more particularly, to a computer system comprising a video interface outputting red, green and blue (RGB) signals after filtering electromagnetic interference (EMI).
2. Description of the Related Art
FIG. 1 is a schematic circuit diagram of a video interface in a conventional computer system.
As shown therein, a video interface comprises a graphic chipset 100, a resistor R100, EMI filters 110a through 110c, and an output port 120.
The graphic chipset 100 is employed as an auxiliary processor to process a video signal, and comprises a digital-to-analog converter (DAC) input terminal, and RGB output terminals.
The DAC input terminal is connected with the resistor R100 having a predetermined resistance. The resistor R100 determines an output level of an RGB signal according to a specification of the graphic chipset 100.
The RGB output terminals output RGB signals characterizing RGB components, forming dots of a display device. A frequency of each RGB signal should be high to display a high-definition picture. Such high-frequency RGB signals cause EMI to be increased. Therefore, the video interface comprises the EMI filters 110a˜110c to filter the EMI from the RGB signals.
Each EMI filter 110a˜110c is a kind of 2π-filter constructed by combination of capacitors C10a˜C30c and inductors L10a˜L20c. Here, various filters such as a π-filter having a capacitor-inductor-capacitor structure, a T-filter constructed by inductor-capacitor-inductor, a C-filter constructed by only capacitor, etc., can replace the 2π-filter as the EMI filter. The EMI filters 110a˜100c vary in a pass-bandwidth and a block-bandwidth corresponding to the frequency of the RGB signals. Further, the EMI filters 110a˜100c have different response patterns corresponding to the frequency of the RGB signals.
The output port 120 is used in transmitting the RGB signals from the EMI filters 110a˜100c to the external display device therethrough.
However, filtering the EMI from the RGB signals causes a phase and amplitude of the RGB signal generated by the graphic chipset 100 to change. Thus, a displayed picture may be decreased in picture quality such as sharpness, etc. Such a problem arises when the display device for a computer system is replaced by another compatible display device. However, in the graphic chipset 100 of the conventional computer system, the resistor R100 and the EMI filters 110a˜110c are invariably connected, so that a user is not allowed to control the video interface in order to improve the picture quality. Further, it is difficult to consider both the EMI and the output level while constructing the EMI filters.